The Most Excellent (Spanish: Excelentísimo Señor (male) or Excelentísima Señora (female), literally "Most Excellent Lord/Lady") is an honorific prefix that is traditionally applied to certain people in Spain and certain Spanish-speaking countries.
Following Spanish tradition, it is an ex officio style (the holder has it as long as they remain in office, in the most important positions of state) and is used in written documents and very formal occasions.
The prefix "The Most Illustrious" (Ilustrísimo/a Señor/a) is the lower version, and is mostly used for non-Grandee titled nobles in Spain and some other officeholders.
Former Presidents and Vice-Presidents of Republics usually retain the style "His/Her Excellency" after finishing their terms, as an honorific.
The more British-style "The Honorable" is preferred for Senators, Representatives and other elected officials.